Electrostatics Chapter 23 Week-1-2
What’s Happening • Clicker use will start on Friday (maybe). • Today we begin the study of charge with make-believe clickers. • There will NOT be a quiz this week. • There WILL be a quiz next Friday. • WebAssigns are now active.
Idiot! If lightening had actually traveled down the kite string, old Ben Franklin would have been toast! Probably never happened, but good story!
History • 600 BC • Thales of Miletos rubs amber (electron in Greek) with cat fur and uses it to pick up feathers. • Important questions: • Why was he doing this? • Didn’t he have a job?? • What happened to the skinned cat?
Amber Million years ago large stands of forests in some parts of the world began to seep globs of sticky resin! This aromatic resin oozed down the sides of trees, as well as filling internal fissures, trapping debris, such as seeds, leaves, feathers and insects. As geologic time progressed the forests were buried and the resin hardened into a soft, warm, golden gem, known as amber. Amber is the fossilized resin of ancient trees which forms through a natural polymerization of the original organic compounds. Most of the world's amber is in the range of 30-90 million years old.
Demo – Lets make some observations Rubber Dead Cat or Rabbit Glass Wool NOT Shown: Aluminum Rods The Tools
Experimental Procedure Pivot • The sequence of Experiments • Identify the two rods • Treat each rod • Bring one rod near to the other • PREDICT WHAT WILL HAPPEN • VOTE ON POSSIBILITIES • Observe what happens • Did we learn anything?
Predictions • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen
Pivot Experiment #1 Rubber rod Rubber rod • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen
Pivot Experiment #2 Rubber rubbed with skin of dead rabbit Rubber rubbed with skin of dead rabbit • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen
Pivot Experiment #3 Glass rubbed with wool Glass rubbed with wool • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen
What is the effect of DISTANCE??
Pivot Experiment #4 Rubber rubbed with skin of dead rabbit Glass rubbed with wool • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen
What’s Going On? • All of these effects involve rubbing two surfaces together. • Or pulling two surfaces apart. • Something has “happened “to each of these objects. • These objects have a new PROPERTY • Other properties are mass, color • We call this NEW PROPERTY CHARGE.
Pivot Experiment #5 Rubber rubbed with skin of dead rabbit Metal Rod • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen
Neutral Object Positive charge attracts negative charges. Rod becomes “polarized. Negative end is closer to positive charge Distance effect causes attraction.
From WebAssign • Three are brought close to each other, two at a time. When objects A and B are brought together, they attract. When objects B and C are brought together, they repel. From this, we conclude which of the following? (Select all that apply.) • [_] Objects A and C possess charges of the same sign. • [_] All three of the objects possess charges of the same sign. • [_] One of the objects is neutral. • [_] Objects A and C possess charges of opposite sign. • [_] We need to perform additional tests to determine information about the charges on the object
Example PITH BALLS
Another Example Volunteer Please
We have also observed that there must be TWO kinds of charge. • Call these two types • positive (+) • negative(-) • We “define” the charge that winds up on the rubber rod when rubbed by the dead cat to be NEGATIVE. • The charge on the glass rod or the dead cat is defined as POSITIVE.
The Triboelectric Series When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged.MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk paper cotton wood acrylic cellophane tape polystyrene polyethylene rubber balloon saran wrap MORE NEGATIVE No! No! No! No!
What have we found? • There are TWO types of charge. • Positive • Negative • Like Charges Attract • Un-Like charges repel • The force between charges increases as they are brought closer together. • This charge separation results from chemical bonds which are severe in the pocess.
Types of Materials • Insulators • “Holds” a charge • Conductors • Allows charge to easily move (current)
This is WAR Ming the merciless this guy is MEAN! • You are fighting the enemy on the planet Mongo. • The evil emperor Ming’s forces are behind a strange green haze. • You aim your blaster and fire … but ……
Nothing Happens! The Green thing is a Force Field! The Force may not be with you ….
Side View The FORCE FIELD Force Big! |Force| o Position
Properties of a FORCE FIELD • It is a property of the position in space. • There is a cause but that cause may not be known. • The force on an object is usually proportional to some property of an object which is placed into the field.
Fields • Imagine an object is placed at a particular point in space. • When placed there, the object experiences a force F. • We may not know WHY there is a force on the object, although we usually will. • Suppose further that if we double some property of the object (mass, charge, …) then the force is found to double as well. • Then the object is said to be in a force field. • The strength of the field (field strength) is defined as the ratio of the force to the property that we are dealing with.
Example – Gravitational Field. • Property is MASS (m). • Force is mg. • Field strength is defined as Force/Property
The Gravitational Field That We Live In. M m mg Mg
Coulomb’s Law – Force between charges • The force between two charges is proportional to the product of the two charges and inversely proportional to the square of the distance between them.
Remember Force is a VECTOR!!
Coulomb’s Law Force is a Vector ! The Unit of Charge is called THE COULOMB Smallest Charge: e ( a positive number) 1.6 x 10-19 Coul. electron charge = -e Proton charge = +e